1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) device, and more particularly to an organic EL device that provides less external light reflection.
2. Description of the Related Art
The organic EL device includes a transparent electrode deposited and patterned on a transparent substrate, which serves as an anode; a functional organic material layer composed of a hole transporting material layer, a light-emissive material layer and an electron transporting material layer successively laminated on the transparent electrode; and a cathode laminated on the functional organic material layer. The cathode may be made of a metal material such as a Mg—Ag alloy or a Al—Li alloy. As a display, the organic EL device employs the EL light extracted externally from the device through the transparent substrate.
The cathode provided on a backside opposite to the direction of externally extracting the light has high reflectivity and gives a strong optical interference effect to the device. Therefore, by optimizing the reflection interference phenomenon of the internal EL light by the backside metallic cathode, the intensity of the EL light externally extracted can be increased.
The backside metallic cathode, however, can increase the EL light intensity, but reflects most of external light incident to the inside of the device. Therefore, the backside metallic cathode greatly deteriorates the visibility of the EL light. In order to avoid such inconvenience, the organic EL device must be provided so as to suppress the reflection of the external light by the backside metallic cathode.
One method conventionally known for suppressing the reflection of the external light is to use a circularly-polarizing filter. However, using the circularly-polarizing filter, which is highly costed, leads to an increase in the production cost. Further, since the circularly-polarizing filter has transparency of about 40% as itself, using the circularly-polarizing filter also attenuates the intensity of the internal EL light externally extracted to about 40%.
Another known method for suppressing the reflection of external light is to employ the optical interference effect so that the reflection of external light is decreased by a low-reflectance laminated structure of an optical thin layer. The low-reflectance laminated structure is disclosed in JP-2001-332391 (particularly on page 5 and in FIG. 3)
JP-2001-332391 discloses the low-reflectance laminated structure composed of aluminum, zinc oxide (ZnO) and aluminum. In particular, in JP-2001-332391, on indium tin oxide (ITO) which is a transparent electrode deposited on a transparent substrate, successively deposited are 4,4′-bis-[(1-naphthyl)-N-phenylamino]-biphenyl (NPD), which serves as a hole transporting layer, and tris(8-quinolinolato-N1, 08)-aluminum (Alq3), which serves as a light emitting layer. Thereafter, lithium fluoride is deposited. On the lithium fluoride layer thus formed, aluminum, zinc oxide (ZnO) and aluminum are successively deposited to provide the low-reflectance structure.
The zinc oxide (ZnO) layer employed in the low-reflectance laminated structure disclosed in JP-2001-332391 is a conductive inorganic layer. Therefore, it is required an evaporation source in producing the above configured organic EL device. Further, the organic layer of the organic EL device is liable to suffer from the damage during the deposition and becomes thermally damaged when the inorganic material such as ZnO is thermally evaporated at a high temperature.
The organic layer also becomes damaged by reflected electrons when a material which requires electron beam deposition (EB deposition) is need to be used. Meanwhile, since the low-reflectance laminated structure serves as a cathode of the organic EL device, the above secondsemitransparent layer must be conductive. For this reason, in the example disclosed in JP-2001-332391, the material which gives less damage to the organic material could not be selected for the low-reflectance laminated structure.